Mupirocin (
Bactroban or
Centany) is an
antibioticIn common usage, an antibiotic is a substance or compound that kills or inhibits the growth of bacteria...
originally isolated from
Pseudomonas fluorescensPseudomonas fluorescens is a common Gram-negative, rod-shaped bacterium. It belongs to the Pseudomonas genus; 16S rRNA analysis has placed P. fluorescens in the P. fluorescens group within the genus, to which it lends its name....
NCIMB 10586, developed by
BeechamBeecham is a British pharmaceutical company. It was once a constituent of the FTSE 100 Index. Beecham, after having merged with SmithKline Beckman, merged with GlaxoWellcome to become GlaxoSmithKline .-History:...
.
Mupirocin is bacteriostatic at low concentrations and bactericidal at high concentrations. It is used topically and is effective against
Gram-positiveGram-positive bacteria are those that are stained dark blue or violet by Gram staining. This is in contrast to Gram-negative bacteria, which cannot retain the crystal violet stain, instead taking up the counterstain and appearing red or pink...
bacteria, including
MRSAMethicillin-resistant Staphylococcus aureus is a bacterium responsible for difficult-to-treat infections in humans. It may also be referred to as multidrug-resistant Staphylococcus aureus or oxacillin-resistant Staphylococcus aureus...
. Mupirocin is a mixture of several pseudomonic acids, with pseudomonic acid A (PA-A) constituting greater than 90% of the mixture. Also present in mupirocin are pseudomonic acid B with an additional hydroxyl group at C8, pseudomonic acid C with a double bond between C10 and C11, instead of the epoxide of PA-A, and pseudomonic acid D with a double bond at C4` and C5` in the 9-hydroxy-nonanoic acid portion of mupirocin.
Figure 1. The structure of pseudomonic acid A-D, labeled A-D, respectively.
Mechanism
Mupirocin has been shown to strongly inhibit
proteinProteins are organic compounds made of amino acids arranged in a linear chain and folded into a globular form. The amino acids in a polymer chain are joined together by the peptide bonds between the carboxyl and amino groups of adjacent amino acid residues...
and
RNARibonucleic acid is a biologically important type of molecule that consists of a long chain of nucleotide units. Each nucleotide consists of a nitrogenous base, a ribose sugar, and a phosphate...
synthesis in
Staphylococcus aureusStaphylococcus aureus is the most common cause of staph infections. It is a spherical bacterium, frequently part of the skin flora found in the nose and on skin. About 20% of the population are long-term carriers of S. aureus. S...
while
DNADeoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms and some viruses. The main role of DNA molecules is the long-term storage of information...
and cell wall formation were also negatively impacted a lesser degree. The inhibition of RNA synthesis was shown to be a protective mechanism in response to a lack of one
amino acidAmino acids are molecules containing an amine group, a carboxylic acid group and one of the twenty R-groups. These molecules are particularly important in biochemistry, where this term refers to alpha-amino acids with the general formula H
2NCHRCOOH, where R is an organic substituent...
,
isoleucineIsoleucine is an α-amino acid with the chemical formula HO2CCHCHCH2CH3. It is an essential amino acid, which means that humans cannot synthesize it, so it must be ingested...
. In vivo studies in
Escherichia coliEscherichia coli , is a Gram negative bacterium that is commonly found in the lower intestine of warm-blooded organisms . Most E...
demonstrated that pseudomonic acid inhibits isoleucine t-RNA synthetase (IleRS). This mechanism of action is shared with
furanomycinFuranomycin is an isoleucine analog....
, an analog of isoleucine
Uses
Mupirocin is used as a topical treatment for bacterial skin infections, for example, furuncle,
impetigoImpetigo is a superficial bacterial skin infection most common among school children. People who play close contact sports such as rugby, American football and wrestling are also susceptible, regardless of age. Impetigo is not as common in adults. It is also highly contagious. The name derives...
, open wounds etc. It is also useful in the treatment of
methicillin-resistant Staphylococcus aureusMethicillin-resistant Staphylococcus aureus is a bacterium responsible for difficult-to-treat infections in humans. It may also be referred to as multidrug-resistant Staphylococcus aureus or oxacillin-resistant Staphylococcus aureus...
(MRSA), which is a significant cause of death in hospitalized patients who have received systemic antibiotic therapy. It is suggested, however, that mupirocin cannot be used for extended periods of time, or indiscriminately, as resistance does develop, and could, if it becomes widespread, destroy mupirocin's value as a treatment for MRSA. It may also result in overgrowth of non-susceptible organisms.
Resistance
Shortly after the clinical use of Mupirocin began, strains of
Staphylococcus aureus that were
resistantDrug resistance is the reduction in effectiveness of a drug in curing a disease or improving a patient's symptoms. When the drug is not intended to kill or inhibit a pathogen, then the term is equivalent to dosage failure or drug tolerance...
to mupirocin emerged. Two distinct populations of mupirocin-resistant
S. aureus were isolated. One strain possessed low-level resistance, MuL, (MIC = 8-256 mg/L) and another possessed high-level resistance, MuH, (MIC > 256 mg/L). Resistance in the MuL strains is probably due to
mutationIn biology, a mutation is a randomly derived change to the nucleotide sequence of the genetic material of an organism.Mutations can be caused by copying errors in the genetic material during cell division, or by exposure to mutagens , or can be induced by the organism itself, by cellular processes...
s in the organism’s wild-type isoleucinyl-tRNA synthetase. In
E. coli IleRS, a single amino acid mutation was shown to alter mupirocin resistance. MuH is linked to the acquisition of a separate Ile synthetase gene, mupA. Mupirocin is not a viable antibiotic against MuH strains. Other antibiotic agents such as
azelaic acidAzelaic acid is a saturated dicarboxylic acid found naturally in wheat, rye, and barley. It is a natural substance that is produced by Malassezia furfur , a yeast that lives on normal skin. It is industrially produced by the ozonolysis of oleic acid...
,
nitrofurazoneNitrofurazone, 2-hydrazinecarboxamide, chemical formula C
6H
6N
4O
4, is a pale yellow crystalline compound....
,
silver sulfadiazineSilver sulfadiazine is a sulfa derivative topical antibacterial used primarily as a topical burn cream on second- and third-degree burns. The cream is kept applied to the burned skin at all times, for the duration of the healing period or until a graft is applied. It prevents the growth of a wide...
, and
ramoplaninRamoplanin is a glycolipodepsipeptide antibiotic drug derived from strain ATCC 33076 of Actinoplanes.-Mechanism:It exerts its bacteriocidal effect by inhibiting cell wall biosynthesis, acting by inhibiting the transglycosylation step of peptidoglycan synthesis.-Uses:Its development has been fast...
have been shown to be effective against MuH strains.
The mechanism of mupirocin differs from other clinical antibiotics rendering cross-resistance to other antibiotics unlikely. However, the MupA gene may co-transfer with other antibacterial resistance genes. This has been observed already with resistance genes for
triclosanTriclosan is a potent wide spectrum antibacterial and antifungal agent. It is a polychloro phenoxy phenol.- Chemical structure and properties :...
,
tetracyclineTetracycline is a broad-spectrum polyketide antibiotic produced by the Streptomyces genus of Actinobacteria, indicated for use against many bacterial infections. It is a protein synthesis inhibitor. It is commonly used to treat acne today, and more recently, rosacea, and played a historical role...
, and
trimethoprimTrimethoprim is a bacteriostatic antibiotic mainly used in the prophylaxis and treatment of urinary tract infections. It belongs to the class of chemotherapeutic agents known as dihydrofolate reductase inhibitors...
.
Biosynthesis of Pseudomonic Acid A
The 74 kb mupirocin
gene clusterA gene cluster is a set of two or more genes that serve to encode for the same or similar products. Because populations from a common ancestor tend to possess the same varieties of gene clusters, they are useful for tracing back recent evolutionary history...
contains six
multi-domainA protein domain is a part of protein sequence and structure that can evolve, function, and exist independently of the rest of the protein chain. Each domain forms a compact three-dimensional structure and often can be independently stable and folded. Many proteins consist of several structural...
enzymes and twenty six other peptides (Table 1). Four large multi-domain type I
polyketide synthase- Background :Polyketide synthases, also known as PKSs, are a family of enzymes or enzyme complexes that produce polyketides, a large class of secondary metabolites, in bacteria, fungi, plants, and animals...
(PKS) proteins are encoded, as well as several single function enzymes with sequence similarity to type II PKSs. It is therefore believed that mupirocin is constructed by a mixed type I and type II PKS system. The mupirocin cluster exhibits an atypical
acyltransferaseAcyltransferase is a type of transferase enzyme which acts upon acyl groups.Examples include:* Glyceronephosphate O-acyltransferase* Lecithin-cholesterol acyltransferase...
(AT) organization, in that there are only two AT domains and both are found on the same protein, MmpC. These AT domains are the only domains present on MmpC, while the other three type I PKS proteins contain no AT domains. The mupirocin pathway also contains several tandem
acyl carrier proteinThe acyl carrier protein is an important component in both fatty acid and polyketide biosynthesis with the growing chain bound during synthesis as a thiol ester at the distal thiol of a 4'-phosphopantethiene moiety...
doublets or triplets. This may be an adaptation to increase the throughput rate or to bind multiple substrates simultaneously.
Pseudomonic acid A is the product of an
esterificationEsterification is the general name for a chemical reaction in which two reactants form an ester as the reaction product. Esters are common in organic chemistry and biological materials, and often have a characteristic pleasant, fruity odor. This leads to their extensive use in the fragrance and...
between the 17C polyketide monic acid and the 9C
fatty acidIn chemistry, especially biochemistry, a fatty acid is a carboxylic acid often with a long unbranched aliphatic tail , which is either saturated or unsaturated...
9-hydroxy-nonanoic acid. The possibility that the entire molecule is assembled as a single polyketide with a Baeyer-Villiger oxidation inserting an
oxygenOxygen Oxygen Oxygen (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter) is the element with atomic number 8 and represented by the symbol O...
into the carbon backbone has been ruled out because C1 of monic acid and C9’ of 9-hydroxy-nonanoic acid are both derived from C1 of acetate.
Monic acid biosynthesis
Biosynthesis of the 17C monic acid unit begins on MmpD (Figure 2). One of the AT domains from MmpC may transfer an activated acetyl group from acetyl-Coenzyme A (CoA) to the first ACP domain. The chain is extended by malonyl-CoA, followed by a SAM-dependent methylation at C12 (see Figure 1 for PA-A numbering) and reduction of the B-keto group to an alcohol. The dehydration (DH) domain in module 1 is predicted to be non-functional due to a mutation in the conserved active site region. Module 2 adds another two carbons by malonyl-CoA extender unit, followed by ketoreduction (KR) and dehydration. Module three adds a malonyl-CoA extender unit, followed by SAM-dependent methylation at C8, ketoreduction, and dehydration. Module 4 extends the molecule with a malonyl-CoA unit followed by ketoreduction.
Assembly of monic acid is continued by the transfer of the 12C product of MmpD to MmpA. Two more rounds of extension with malonyl-CoA units are achieved by module 5 and 6. Module 5 also contains a KR domain.
Figure 2. The domain structure of MmpA, MmpC, and MmpD for the synthesis of monic acid. The biosynthesis of monic acid is not colinear but has been rearranged in this diagram. The protein name is displayed inside of the arrow with module and domain structure listed below. ACP=acyl carrier protein, AT=acyl transferase, DH=dehydratase, ER=enoyl reductase, HMG=3-hydroxy-3-methylglutaric acid, MeT=methyl transferase, KR=ketoreductase, KS=ketosynthase, TE=thioesterase.
Post-PKS tailoring
The keto group at C3 is replaced with a methyl group in a multi-step reaction (Figure 3). MupG begins by decarboxylating a malonyl-ACP. The alpha carbon of the resulting acetyl-ACP is linked to C3 of the polyketide chain by MupH. This intermediate is dehydrated and decarboxylated by MupJ and MupK, respectively.
Figure 3. The C15 methyl group of monic acid is attached to C3 by the following reaction scheme. MupH is an 3-hydroxy-3-methylglutarate-Coenzyme A synthase, MupJ and MupK are Enoyl-CoA hydratases.
The formation of the
pyranIn chemistry, a pyran is a six membered heterocyclic ring consisting of five carbon atoms and one oxygen atom and containing two double bonds. The molecular formula is C5H6O. There are two isomers of pyran that differ by the location of the double bonds...
ring requires many enzyme mediated steps (Figure 4). The double bond between C8 and C9 is proposed to migrate to between C8 and C16.
Gene knockoutA gene knockout is a genetic technique in which an organism is engineered to carry genes that have been made inoperative . Also known as knockout organisms or simply knockouts, they are used in learning about a gene that has been sequenced, but which has an unknown or incompletely known function...
experiments of mupO, mupU, mupV, and macpE have eliminated PA-A production. PA-B production is not removed by these knockouts, demonstrating that PA-B is not created by hydroxylating PA-A. A knockout of mupW eliminated the pyran ring, identifying MupW as being involved in ring formation. It is not known if this occurs before or after the esterification of monic acid to 9-hydroxy-nonanoic acid.
Figure 4. The pyran ring of mupirocin is generated in this proposed multistep reaction [15]. Gene knockouts of mupO, mupU, mupV and macpE abolish PA-A production but not PA-B production, demonstrating that PA-B is a precursor to PA-A.
The
epoxideAn epoxide is a cyclic ether with three ring atoms. This ring approximately defines an equilateral triangle, which makes it highly strained. The strained ring makes epoxides more reactive than other ethers. Simple epoxides are named from the parent compound ethylene oxide or oxirane, such as in...
of PA-A at C10-11 is believed to be inserted after pyran formation by a cytochrome P450 such as MupO. A gene knockout of mupO abolished PA-A production but PA-B, which also conatins the C10-C11 epoxide, remained. This indicates that MupO is either not involved or is not essential for this epoxidation step.
9-Hydroxy-nonanoic acid biosynthesis
The nine carbon fatty acid 9-hydroxy-nonanoic acid (9-HN) is derived as a separate compound and later esterified to monic acid to form pseudomonic acid.
13C labeled
acetateAn acetate, or ethanoate, is either a salt or ester of ethanoic acid.In chemistry, the abbreviation Ac refers to the acetyl group. The anion and the functional group may be written as −OAc and AcO−, or OAc respectively...
feeding has shown that C1-C6 are constructed with acetate in the canonical fashion of
fatty acid synthesisFatty acids are formed by the action of Fatty acid synthases from acetyl-CoA and malonyl-CoA precursors.-De Novo Synthesis in Humans:In humans fatty acids are predominantly formed in the liver and adipose tissue, and mammary glands during lactation. Most acetyl-CoA is formed from pyruvate by...
. C7’ shows only C1 labeling of acetate while C8’ and C9’ show a reversed pattern of 13C labeled acetate. It is speculated that C7-C9 arises from a 3-hydroxypropionate starter unit, which is extended three times with malonyl-CoA and fully reduced to yield 9-HN. It has also been suggested that 9-HN is initiated by 3-hydroxy-3-methylglutaric acid (HMG). This latter theory was not supported by feeding of [3-
14C] or [3,6-
13C
2]-HMG.
MmpB is proposed to catalyze the synthesis of 9-HN (Figure 5). MmpB contains a KS, KR, DH, 3 ACPs, and a thioesterase (TE) domain. It does not contain an enoyl reductase (ER) domain which would be required for the complete reduction to the nine carbon fatty acid. MupE is a single domain protein that shows sequence similarity to known ER domains and may complete the reaction. It also remains possible that 9-hydroxy-nonanoic acid is derived partially or entirely from outside of the mupirocin cluster.
Figure 5. MmpB is proposed to synthesize 9-HN with a 3-hydroxy-propionate starter unit and three malonyl-CoA extenter units. The domain structure of MmpB is shown below along side with MupE, the proposed enoyl reductase required for complete saturation of 9-HN. ACP=acyl carrier protein, DH=dehydratase, ER=enoyl reductase, KR=ketoreductase, KS=ketosynthase, TE=thioesterase.
Sources
The original manufacturer's (
GlaxoSmithKline GlaxoSmithKline plc is a United Kingdom-based pharmaceutical, biological, and healthcare company. GSK is the world's second largest pharmaceutical company and a research-based company with a wide portfolio of pharmaceutical products covering anti-infectives, central nervous system, respiratory,...
) product is Bactroban. It is now available as a
genericA generic drug is a drug which isproduced and distributed without patent protection. The generic drug may still have a patent on the formulation but not on the active ingredient....
. One source is manufactured by
Ortho-McNeil PharmaceuticalThe company was formed from the merger of Ortho Pharmaceutical and McNeil Pharmaceutical in 1993. Both of these pharmaceutical companies are pioneers and leaders in areas such as pain management, acid reflux disease, and infectious diseases...
called Centany. Another is from
Teva Pharmaceutical IndustriesTeva Pharmaceutical Industries Ltd. , is an international pharmaceutical company headquartered in Petah Tikva, Israel. It specializes in generic and proprietary pharmaceuticals and active pharmaceutical ingredients. It is the largest generic drug manufacturer in the world and one of the 20 largest...
which is simply called Mupirocin Ointment.